The present invention relates to locked joints between metal pipes, especially spheroidal-graphite cast-iron pipes.
The joints between such pipes are subjected to separation forces when fluids under pressure are conveyed.
One particularly economical way of locking these joints consists in incorporating, into the seals made of elastomer or other flexible substance, a ring of locking inserts of planar general shape which are contained in radial planes. Where the joints tend to separate, the inserts are braced against appropriate bearing surfaces and thus prevent the pipes from separating. An example of this technique is described in the Assignee's published Patent Application FR-2,679,622.
When the pipes are metallic, it is sometimes necessary, for reasons of safety, to insulate the pipes electrically from each other. Now, the inserts, being metallic, form electrically conducting pathways.
It has been proposed to produce inserts from insulating materials, but this has turned out to lead to expensive and fragile inserts.
It has also been proposed to coat metal inserts with an insulating layer, at least in a region where these inserts bear against the pipes. However, such coatings are in contact with a relatively rough metal surface, are moved frictionally over this surface and are stressed not only in compression but also in shear. As a consequence, they are quickly damaged and removed.